Method for monitoring and controlling a laser cutting process

ABSTRACT

An example device for monitoring and controlling a laser cutting process on a workpiece includes an image capturing apparatus for capturing an image of a region of the workpiece to be monitored, in which the region of the workpiece to be monitored includes a region of interaction of a laser beam with the workpiece, and an evaluation apparatus for detecting material boundaries of the workpiece using the captured image. The evaluation apparatus is configured to determine at least one characteristic value of the laser cutting process based on a geometric relationship between at least two of the detected material boundaries, the region of interaction, or combinations thereof.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of and claims priority under 35U.S.C. § 120 to U.S. application Ser. No. 15/419,190, filed Jan. 30,2017, which is a continuation of U.S. application Ser. No. 13/961,596,filed Aug. 7, 2013, which is a continuation of PCT Application No.PCT/EP2012/051634 filed on Feb. 1, 2012, which claimed priority toGerman Application No. 10 2011 003 717.9 filed on Feb. 7, 2011. Thecontents of these priority applications are hereby incorporated byreference in their entirety.

TECHNICAL FIELD

The present disclosure relates to a device for monitoring and forcontrolling a laser cutting process on a workpiece.

BACKGROUND

An example of a device for monitoring a laser cutting process isdisclosed in DE 10 2005 024 085 A1. To monitor a laser machiningprocess, the device described in that reference has, inter alia, acamera and an imaging apparatus, which images the region from the zoneof interaction or the region of interaction between laser beam andworkpiece to be observed on the camera. The output signals from thecamera are fed to an evaluation circuit, which processes both thesignals from the camera and the signals from a radiation-sensitivereceiver, and are used to characterize the course of the laser machiningoperation. Here, the radiation-sensitive receiver and the camera cancover different spectral ranges. No further information is given aboutthe features used for characterization or the specific evaluationthereof.

The reference WO 91/04828 likewise discloses a monitoring apparatus, inwhich a camera arranged on the laser machining head, coaxially with theoptical axis of a laser beam guided in the direction of the workpiece,is used for focal position determination during a laser cutting process.Here, the camera detects a zone of interaction between laser beam andworkpiece and, by using the width of the zone of interaction,conclusions are drawn about the focal position or about the distancebetween laser machining head and workpiece.

The reference DE 10 2008 051 459 A1 discloses a further such monitoringapparatus which, in particular, is used for edge detection during thelayer by layer machining of bodies by means of laser radiation. Thedevice comprises an imaging detector for forwarding a digital image,converted into gray stages or true-color/color-coded, to a dataprocessing system.

The reference DE 43 36 136 C2 describes a method for laser machining inwhich the laser light reflected at the workpiece, together withgenerated secondary light, passes back to a laser oscillator and there,with the aid of a mirror, some of the laser light and of the secondarylight is separated off. The secondary light is captured by an opticalsensor, separately from the laser light component, and a control signalfor controlling the laser machining is derived from the remainingsecondary light component. In one exemplary embodiment, the surface ofthe workpiece is irradiated and the reflected radiation passing througha nozzle opening is detected in order to determine a cutting path orcutting point during the laser cutting. The position of the cuttingpoint is compared with the center of the nozzle in order to control thelaser cutting process such that the position of the cutting pointcoincides with the center of the nozzle. In addition, by using theobservation of the nozzle opening, the deformation or a blockage of thenozzle opening is determined.

Furthermore, in relation to nozzle eccentricity, the reference EP 1 728581 A1 discloses a device and a method for aligning a laser beam withthe nozzle center, in which an image of an illuminated nozzle and afocused laser beam respectively are captured and related to each othervia an image evaluation unit.

Furthermore, in order to detect material burn-up, an example ofmonitoring the shaping of the cutting front (“red heat region”) is knownfrom the reference JP 07116885. If the latter expands, the systemchanges over to using an inert gas as cutting gas instead of oxygen. Forthe same purpose or to distinguish between correct and incorrectmachining, the reference JP 11320149 discloses an assessment by using acomparison between captured optical signals. In the reference DE 101 29751, in order to detect material burn-up, the temperature of theworkpiece in the vicinity of the cut is monitored by using infraredtemperature measuring apparatus and is compared with a temperaturelimit.

On the basis of the foregoing references, cited by way of example, itbecomes clear that the capturing and evaluation of a multiplicity ofcharacteristics determining the quality of the laser cutting process onthe basis of devices and methods based on various capturing andevaluation principles is very complex. This relates both to thestructure of the device itself and to the signal processing.

However, via the devices described above and the associated methods forevaluating the process images captured, no complete image which would besuitable for characterizing the entire laser cutting process results. Inparticular, the cut quality itself is inadequately reproduced andcontrol is not carried out comprehensively over the entire processcourse. In this connection, the entire process course is understood notonly as the cutting operation as such but it is also possible for theprocess course to comprise both the piercing operation and also aplurality of successive laser cuts within the context of a processsequence.

SUMMARY

The present disclosure relates to devices for monitoring and forcontrolling a laser cutting process on a workpiece, in which the devicesinclude: an image capturing apparatus for capturing an image of a regionof the workpiece to be monitored, which in particular comprises a regionof interaction of a laser beam with the workpiece, and an evaluationapparatus for detecting material boundaries, in particular edges of theworkpiece, by using the captured image. The disclosure also relates tomethods for monitoring, in particular for controlling, a laser cuttingprocess, comprising the steps of: capturing an image of a region of theworkpiece which is to be monitored, which in particular comprises aregion of interaction of laser beam with the workpiece, and evaluatingthe captured image in order to detect material boundaries, in particularedges, of the workpiece

The devices and methods disclosed herein permit the serial or parallelcapturing and evaluation of a large number of features whichcharacterize a laser cutting process.

According to an aspect of the disclosure, a device includes anevaluation apparatus configured to determine a characteristic value, inparticular a cut quality, of the laser cutting process by using ageometric relationship between at least two of the detected materialboundaries and/or by using the region of interaction.

In order to monitor the laser cutting process, it is proposed to capturean image of a detail (i.e. of a monitored region) of the workpiece,which typically can comprise the zone of interaction between the laserbeam and the workpiece during a piercing operation or a cuttingoperation, i.e. during a relative movement between laser beam andworkpiece, and a cut gap that is forming or has already been formed. Theevaluation unit can detect two or more material boundaries by using thecaptured image and, by using a geometric relationship between thematerial boundaries, determine at least one characteristic value of thecutting process. Additionally or alternatively, given a suitable choiceof the detected wavelength range, for example in the near infrared (NIR)range, a thermal image or process autoluminescence of the monitoredregion, in particular of the zone of interaction, can be captured andthe evaluation apparatus can determine at least one characteristic valueof the laser cutting process by using the thermal image or the processautoluminescence. Detection of the process luminescence is also possiblewith the aid of UV radiation, in this case the radiation originatingfrom a plasma generally being detected.

In particular, in the device the characteristic values can be determinedwith the aid of one and the same capturing and evaluation logic unit, sothat the structure of the device and the performance of the method aresimplified. Here, the evaluation apparatus is designed or programmed todetermine or to calculate the characteristic values by using the datasupplied by the capturing apparatus.

In the device, by using a few process-induced geometric features thatcan be captured by the capturing unit or the zone of interaction, amultiplicity of variables or process features to be used for the processmonitoring and/or control can be determined with the aid of theevaluation unit. The characteristic values supplied by the evaluationunit can be used for the control of the laser machining process via anassessment which can be carried out both in the evaluation unit itselfand also in a logic unit (e.g. a control apparatus) connected downstreamthereof.

As characteristic values, it is possible to determine, for example:crater formation during the piercing operation, gap width, failed cut,and material burn-up (self-burning) during the cutting process, cutquality (burr formation) during the cutting process, and disruptiveinfluences, for example as a result of inadequate nozzle spacing andnon-process-synchronous switching on and/or switching off of the laserbeam. These quality-determining characteristic values for the processmonitoring and/or control also include the cutting front angle. Thedetermination of the aforementioned characteristic values will bedescribed in detail below.

In some embodiments, the evaluation apparatus is designed to detect cutedges of a cut gap as material boundaries formed during the lasercutting process and to determine a gap width of the cut gap ascharacteristic value. The detected cut edges typically run parallel toeach other, so that the gap width, i.e. the distance between the cutedges, is substantially constant and can be determined in astraightforward way.

In further embodiments, the evaluation apparatus is designed to detectmaterial burn-up of the workpiece if a predefined gap width of the cutgap is exceeded and/or in the event of too rapid an increase in the gapwidth of the cut gap. Material burn-up leads to a widening of the cutedges and of the cutting front which can possibly be so great that,within the monitored detail (e.g. in the event of imaging through acutting gas nozzle), the cut edges can no longer be detected but onlythe (substantially semicircular) cutting front. If the danger ofmaterial burn-up is detected in good time, e.g. by comparing the cut gapwidth with a reference value which should not be exceeded, suitablecountermeasures can be taken, e.g. the supply of oxygen can beinterrupted or reduced in order to counteract the material burning.Further possibilities for the early detection of material burn-up willbe presented further below.

In further embodiments, the evaluation apparatus is designed to detect afailed cut if the cut gap falls below a predefined gap width.Alternatively or additionally, a failed cut can be detected by means ofthe comparison of the area of the observed cutting front with areference area, which corresponds to the area of the cutting front in agood or quality cut. A failed cut can also be detected if the radiationintensity emitted by the reference area exceeds a limiting value for thereference brightness in the case of a normal cut. This limiting valuehas previously been determined empirically by the brightness of thereference area having been measured during a laser cutting process inwhich a good cutting result was achieved.

In further embodiments, the evaluation apparatus is designed to detectcut edges formed during the laser cutting process as material boundariesand to determine a gap center of the cut gap as characteristic value.The gap center can be determined by the variable distance between thecut edges being determined at several points perpendicular to the feeddirection. Determining the cut gap width at several points along the cutgap is beneficial since, even during a laser cutting process, a(possibly not entirely avoidable) change in the cut gap width can occur,for example when a changeover is made between different cuttingconditions (e.g. between large and small contours).

In some implementations, the evaluation apparatus is designed todetermine a gap center in a further cut gap which does not run parallelto the first cut gap and, by using the two gap centers, to determine atool center point of the laser cutting process in a plane parallel tothe workpiece. The tool center, also called the tool center point (TCP),is used as a reference point, in particular as an origin, for a toolcoordinate system. The TCP can be used in particular to define geometricrelationships, in particular distances, between the tool (lasermachining head) and material boundaries of the workpiece.

In some implementations, the evaluation apparatus is designed todetermine a geometric relationship between the tool center point and atleast one detected material boundary, and the device has a controlapparatus for switching on or switching off the laser beam as a functionof the determined geometric relationship. By using the geometricrelationship, in particular the distance, between TCP and materialboundary, it is possible to define the time at which the laser beam isswitched on and off, so that the cut start and the cut end can be madeat the desired position on the workpiece.

In some implementations, the evaluation apparatus is designed todetermine a geometric relationship between the tool center point and thecut edges, in particular the gap center, of a cut gap formed during apreceding laser cutting process, the control apparatus preferably beingdesigned to switch on or switch off the laser beam when the gap centeris reached by the tool center point. In this way, it is possible toproduce a smooth transition or connection between contours which are cutduring successive laser cutting processes.

In some implementations, the evaluation apparatus is designed todetermine a geometric relationship between the tool center point and anedge of the workpiece, and the control apparatus is preferably designedto switch on or switch off the laser beam when the edge is reached bythe tool center point. Here, the edge is typically formed at the outeredge of the (plate-like) workpiece. As a result of the timely switchingoff and switching on of the laser beam when the edge is reached, it isensured that no cut is made beyond the edge of the workpiece.

In further embodiments, the evaluation apparatus is designed to detect acutting front upper edge of a workpiece surface facing the incidentlaser beam and a cutting front lower edge of a workpiece surface facingaway from the incident laser beam as material boundaries and, fromthese, by taking into account the thickness of the workpiece, todetermine a cutting front angle of the laser cutting process ascharacteristic value. The cutting front angle in a laser cutting processdepends on several cutting parameters, in particular on the feed orcutting speed. If the cutting front angle deviates from a referencevalue or a reference range, this can point to a cutting defect, whichcan be corrected by suitable measures, for example adaptation of thecutting speed.

In some implementations, the evaluation apparatus is designed to capturean outer boundary and an inner boundary of a pierced hole on theworkpiece as material boundaries during a piercing operation and todetermine the formation of a crater on the workpiece as characteristicvalue. Both the chemical material composition and the surface finish ofthe workpiece, which can vary from manufacturer to manufacturer, have asubstantial influence on the piercing operation. In particular, in thecase of high metal sheet thicknesses (e.g. more than 15 mm), thepiercing operation can therefore be disrupted in the event of anunfavorable material finish, in such a way that the laser beam does notdrill a narrow hole but, on account of overheating and an exothermiciron-oxygen reaction that subsequently proceeds, a large conical crateris formed. The material boundaries thereof can be captured and, when alimiting value for the distance between the boundaries is exceeded,countermeasures can be initiated.

In further embodiments, the image capturing apparatus is designed tocapture the region to be monitored coaxially with respect to a laserbeam axis. Coaxial capturing of the region to be monitored is possibleindependent of direction.

In some embodiments, a distance between an image plane of the imagecapturing apparatus for capturing the image and an imaging and focusingoptics can be varied, and the evaluation apparatus is designed, by usingthe detection of at least one material boundary of the workpiece with afirst distance between the image plane and the imaging optics and thedetection of an inner contour of a nozzle for the passage of the laserbeam onto the workpiece as a material boundary with a second distancebetween the image plane and the imaging optics, to determine a distancebetween the nozzle and the workpiece. The optics assigned to the imagecapturing apparatus are used in this case for the sharp imaging of twoobjects arranged at different distances from the image plane of theimage capturing apparatus, specifically by a distance between the optics(e.g., lens) and the image plane being varied. Here, the contours areonly detected by the evaluation apparatus if the material boundary to becaptured (nozzle or workpiece) is located within the range of the depthof focus of the optics. By using the displacement travel of the opticswhich is necessary to image the respective material boundary sharply,the distance between nozzle and workpiece can be determined. Ifnecessary, a respective contour can be detected not only with onedistance between image plane and optics but within a distance interval.In this case, in order to determine the nozzle distance, that distancefrom the interval is chosen at which the respective contour is sharpest,i.e. can be detected best.

In further embodiments, the evaluation apparatus is designed todetermine the presence or lack of burr formation on the cut gap andtherefore a cut quality as characteristic value of the laser cuttingprocess by using the image, in particular by using the thermal image orthe process autoluminescence in the NIR/IR range, of the region ofinteraction. The region of interaction or the geometry thereof can beobserved here by means of the image capturing apparatus via a suitablewavelength filter which is transparent to wavelengths for example in thenear infrared range or in the UV range (for the detection of the plasmaluminescence above the region of interaction). The image capturingapparatus can have different detectors for capturing the materialboundaries and the region of interaction. Use of a single detector, e.g.a (CCD) camera, in conjunction with a suitably adjustable wavelengthfilter for capturing both the material boundaries and the region ofinteraction is, however, both space-saving and inexpensive.

In further embodiments, the evaluation apparatus is designed, during aflame cutting process (in particular when cutting constructional steel),upon the occurrence of a local intensity minimum of the image in theregion of the cutting front, to conclude that there is a good cut, inparticular a lack of burr formation. In particular, in a laserbeam-guided flame process, a local intensity minimum (radiation sink)occurs with respect to the environment in the region of the cuttingfront in the case of a good cut, i.e. with smooth cut edges without anyburr formation. This local minimum typically disappears when burrformation occurs at the cut edges.

In further embodiments, the evaluation apparatus is designed, during amelting cutting process (in particular when cutting stainless steel), byusing the lack of a repeating fluctuation in the radiation intensity ofthe (thermal) image or the process autoluminescence in the NIR/IRwavelength range in the region of the cut gap and/or upon the occurrenceof three luminescent stripes originating from the cutting front, toconclude that burr formation is present. If no sporadically occurringflickering in the kerf is detected in the wake, this points to theproduction of whisker burr, in which the entire melt volume feeds thewhisker burr, so that no flying sparks are produced which would manifestthemselves as flickering in the wake. The presence of burr formation canalso be detected by using two bright luminescent stripes directedrearward in the region of the two cut edges and a further luminescentstripe which, typically, runs in the center between the two outerluminescent stripes. Here, the luminescent stripes are generallycomparatively long, which points to the occurrence of an azimuthal meltflow and therefore to the production of burr in the form of crumb burr,i.e. far back in the wake.

In some embodiments, the evaluation apparatus is designed to detectstriations on at least one cut edge of a cut gap formed during the lasercutting process and, by using a frequency of the striations, to drawconclusions about material burn-up. The striations can be detected wellby using the thermal image or the process autoluminescence but canpossibly also be detected in the visible range as material boundaries.The frequency of the striations typically decreases in the region ofthat cut edge at which material burn-up is immediately imminent, so thatsuitable countermeasures can be initiated even before the occurrence ofthe material burn-up.

In further embodiments, the evaluation apparatus is designed to drawconclusions about material burn-up by using a rise in the totalintensity and/or by using a fluctuation in the total intensity of theimage of the region of interaction. The total intensity (typicallydetected through the nozzle opening) of the detected radiation (in thenear infrared/infrared (NIR/IR range)) rises, since the zone ofinteraction increases in the event of material burn-up. Additionally oralternatively, material burn-up can also be determined by using anincreased fluctuation in the total measured brightness value as comparedwith a conventional cutting process.

A further aspect of the disclosure relates to a method which includes:determining at least one characteristic value of the laser cuttingprocess, in particular a cut quality, by using a geometric relationshipbetween at least two of the detected material boundaries and/or by usingthe region of interaction. In the method for monitoring a laser cuttingprocess, in particular the embodiments and developments describedfurther above in conjunction with the device and with the evaluationapparatus can be implemented as further method steps, which will not bediscussed in more detail below, for the purpose of simplification. Inparticular, it is also possible for the characteristic values of thelaser cutting process cited further above to be determined in themethod.

Further advantages of the disclosure can be gathered from thedescription and the drawing. Likewise, the features mentioned above andthose cited further on can each be used on their own or in a pluralityin any desired combination. The embodiments shown and described are notto be understood as a final enumeration but instead have an exemplarycharacter for the description of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic illustration of an embodiment of a deviceaccording to the an aspect of the disclosure for monitoring and forcontrolling a laser cutting process, that includes an image capturingunit,

FIG. 2 shows an illustration of an image of the workpiece, captured bythe image capturing unit, by which several characteristic values of thecutting process are determined,

FIG. 3 shows an illustration of an image of the workpiece, by which acut end on a sheet-metal edge of the workpiece is detected,

FIG. 4 shows an illustration of an image of the workpiece, by which acut end on an already cut contour is detected,

FIG. 5 shows a further illustration of an image of the workpiece, bywhich a cut start is detected,

FIG. 6 shows an illustration of an image of the workpiece during apiercing operation,

FIGS. 7A, 7B and 7C show illustrations of a thermal image of themonitored region of the workpiece during melt cutting and in thepresence of a cut (FIG. 7A), a crumb burr (FIG. 7B) and a whisker burr(FIG. 7C), and

FIG. 8 shows an illustration of a thermal image during flame cutting inthe presence of a cut.

DETAILED DESCRIPTION

FIG. 1 shows an exemplary structure of a device 1 for process monitoringand control of a laser cutting process on a workpiece 2 by means of aCO₂ laser machining installation, of which only a machining unit 3 (partof a laser machining head) having a focusing lens 4 made of zincselenide for focusing a CO₂ laser beam 5 of the laser machininginstallation, a cutting gas nozzle 6 and having a deflection mirror 7 isillustrated in FIG. 1. In the present case, the deflection mirror 7 isdesigned to be partly transparent and therefore forms a component on theinlet side of the device 1 for process monitoring.

The deflection mirror 7 reflects the incident CO₂ laser beam 5 (having awavelength of about 10 μm) and transmits the radiation 8 that isrelevant to the process monitoring, is reflected from the workpiece 2and emitted from the zone of interaction in a wavelength range which, inthe present example, lies between about 550 nm and 2000 nm. As analternative to the partially transparent deflection mirror 7, a scrapermirror or a perforated mirror can also be used in order to feed theprocess radiation 8 to the device 1. However, the use of a scrapermirror typically leads to part of the process radiation being masked outand to the unmodified beam diameter being limited. The use of aperforated mirror generally leads to diffraction effects of the processradiation and to a strong influence on the CO₂ laser radiation.

In the device 1, after the partially transparent mirror 7 there isarranged a further deflection mirror 9, which deflects the processradiation 8 onto a geometrically highly resolving camera 10 as imagecapturing unit. The camera 10 can be a high-speed camera, which isarranged coaxially with respect to the laser beam axis 11 or to theextension of the laser beam axis 11 a and thus directionallyindependently. In principle, there is the possibility of capturing theimage by means of the camera 10 in the incident light method as well,i.e. in the visible wavelength range, possibly also in the NIRwavelength range, if an additional illumination source which emits inthe NIR range is provided and, alternatively, the capturing of theprocess autoluminescence in the UV and NIR/IR wavelength ranges.

For improved imaging, in the present example, between the partiallytransparent mirror 7 and the camera 10 there is provided an imagingfocusing optical system 12, depicted as a lens in FIG. 1, which focusesthe radiation 8 that is relevant to the process monitoring on the camera10. By means of an aspherical form of the imaging optical system or thefocusing lens 12, aspherical aberrations during the imaging can beprevented or at least reduced.

In the example shown in FIG. 1, a filter 13 in front of the camera 10 isadvantageous if further radiation or wavelength components is/are to beexcluded from the capturing with the camera 10. The filter 13 can beformed, for example, as a narrowband bandpass filter with a low fullwidth at half maximum in order to avoid or to reduce chromaticaberrations. The position of the camera 10 and of the imaging opticalelement 12 and/or the filter 13, present in the present example, alongthe laser beam axis 11 can be adjusted and varied as required by apositioning system 14 known to those skilled in the art, illustrated bya double arrow for the purpose of simplification.

In the present example, the camera 10 is operated in the incident lightmethod, i.e. an additional source of illumination 15 is provided abovethe workpiece 2 and, via a further partially transparent mirror 16,couples illumination radiation 17 into the beam path, coaxially withrespect to the laser beam axis 11. As an additional source ofillumination 15, laser diodes, for example having a wavelength of 658nm, or diode lasers, for example having a wavelength of 808 nm, can beprovided and can be arranged coaxially, as shown in FIG. 1, but alsooff-axis with respect to the laser beam axis 11. The additional sourceof illumination 15 can also, for example, be arranged outside (inparticular beside) the machining unit 3 and be aimed at the workpiece 2;alternatively, the source of illumination 15 can be arranged inside themachining unit 3 but not aimed at the workpiece 2 coaxially with respectto the laser beam 5.

As shown in FIG. 2, the camera 10 captures a highly resolved image 20 ofa region 21 (detail) to be monitored of the workpiece 2. The image 20 isdelimited by the circular inner contour 6 a (cf. FIG. 1) of the nozzle6.

In the example illustrated in FIG. 2, the image 20 shows the region 21to be monitored during a laser melt cutting process, in which theworkpiece 2 is moved in a feed direction V_(sheet) relative to thenozzle 6 and to the machining unit 3 (laser machining head).Alternatively or additionally, the relative movement between theworkpiece 2 and the nozzle 6 or the machining unit 3 (laser machininghead) can be executed by the movement of the nozzle 6 or the machiningunit 3. During the melt cutting process, a region of interaction 22, 23is formed between the laser beam 5 and the workpiece 2, which regioncomprises a heat forerunner zone 22 and a cutting front 23, which areadjoined in the feed direction V_(sheet) by a cut gap 24 (alsodesignated kerf below).

An evaluation apparatus 18 shown in FIG. 1 is used to evaluate the image20 and in particular to detect material boundaries within the monitoredregion 21 on the upper side 2 a and the underside 2 b of the workpiece2. The evaluation apparatus 18 has a signal connection to a controlapparatus 19, likewise shown in FIG. 1, which controls and regulates thelaser cutting process, specifically as a function of characteristicvalues of the laser cutting process determined by the evaluationapparatus 18.

Amongst other things, by using the camera image 20, the followingfeatures of a laser cutting process can be determined by the evaluationunit 18 in order to determine characteristic values: material boundarieson the workpiece upper side 2 a and underside 2 b, in particular edgesof the workpiece, nozzle edge and nozzle center of the laser machiningnozzle, geometric dimensions of the kerf (not only opposite cut edgesbut also the region of the zone of interaction, e.g. of the cuttingfront), position of the kerf relative to the nozzle edge/center, orposition of already cut regions relative to the current cuttingposition, among other features. The detection of these and furtherfeatures for determining characteristic values of the laser machiningprocess will be described in more detail below.

In the example shown in FIG. 2, as characteristic value, the gap widthA2 of the kerf 24 is determined on the basis of the high resolutioncamera image 20, by the evaluation unit 18 detecting the cut edges K1.1and K1.2 of the kerf 24 and the spacing thereof, which coincides withthe cut gap width A2. In the case of a laser cutting process, the cutedges K1.1, K1.2 generally run (virtually) parallel, so that the cut gapwidth A2 is (virtually) constant, in particular in a good cut.

The evaluation unit 18 itself or logic unit connected downstreamthereof, e.g. the control apparatus 19, is able to determine, via thecomparison with a reference cut width that is defined previously andstored for comparison, whether for example when the cut gap width fallsbelow a minimum A2 _(min), a failed cut is present, i.e. a complete lackof a kerf, or when the cut gap width exceeds a maximum A2 _(max),material burn-up (self-burning) is present or, in the case of an oxygenflame cut of structural steel, washouts (pits) are present.

Alternatively or additionally, material burn-up can also be determinedvia the (time) change in the cut gap width A2—both with regard to anabsolute change and via the rate of change. A number of evaluationmethods relating to the gap width can also be used in parallel. Materialburn-up leads to a widening of the cut edges K1.1, K1.2 and the cuttingfront 23, which can possibly become so large that the cut gap 24 becomeswider than the nozzle opening 6 a or the nozzle contour K3, so that thecut edges K1.1, K1.2 in the monitored region 21 are no longerdetectable. Here, the image 20 shown in FIG. 2 of the cut gap 24 withthe virtually parallel cut edges K1.1, K1.2 changes to aquasi-semicircular cutting front and the detail 21 of the workpiece 2exhibits only a radius corresponding to the cutting front. In the eventof material burn-up, the cutting front does not end directly with thelaser beam 5 either but is displaced in front of the latter since, inthis case, the cutting gas dominates the burning process.

Alternatively or additionally to falling below the minimum spacing A2_(min), a failed cut can be detected by using an area F2 which is formedbetween a front edge K2.1 and a rear edge K2.2 of the detected cuttingfront 23. For this purpose, the area F2, which corresponds to theprojection of the cutting front 23 in the XY plane, is related to areference area. A failed cut is present if the area F2 reaches the sizeof the reference area, the ratio reference area/F2 is therefore equal toone. The reference area in this case corresponds to the area of theprojected cutting front in the case of a good cut, i.e. in the case of acut with good cutting quality. A failed cut can additionally be detectedif the brightness of the cutting front 23 is greater than in the case ofa reference good cut, it being possible for the luminescence to occurcontinuously and/or sporadically and the luminescent area beingapproximately equal to or greater than the kerf width A2.

By using an image 20, as illustrated in FIG. 2, pits during the oxygenflame cutting of structural steel can also be detected if these beginfrom above (i.e. from the workpiece upper side 2 a), specifically byusing a non-periodic increase in the cut gap width A2 or by using the(at least temporary) loss of parallelism between the cut gap edges K1.1,K1.2. When observing process luminescence, as will be described below inconjunction with FIGS. 7A-C and FIG. 8, pits can also be detected via adrop in brightness and via the occurrence of flashes, i.e. short andintensive increases in the brightness in the region of the cutting front23, to be specific typically in a punctiform manner on the outside ofthe cutting front, in the region of the transition to the parallel cutedges K1.1, K1.2.

In addition to the determination of a failed cut or material burn-up orpits, via the kerf width A2 it is also possible to determine theposition of the tool center P2, which will also be designated TCP (ToolCenter Point) below. The position of the latter in the Y direction isdefined by a center line 25, which extends parallel and centrally inrelation to the opposite cut edges K1.1, K1.2 of the kerf 24. Theposition of the TCP P2 can additionally also be determined in the Xdirection, specifically as a center line between the edges K5.1, K5.2 ofa further kerf 27 which, in the example shown in FIG. 2, has beenproduced in a preceding laser cutting process (with feed direction inthe Y direction). By using the two kerfs 24, 27, the tool center pointP2 can be defined unambiguously in the XY plane (parallel to theworkpiece surface). In principle, instead of the kerf 27 shown extendingat right angles to the kerf 24, any other kerf not extending parallel tothe kerf 24 can be used to determine the tool center point P2 in the XYplane.

The tool center point P2 determined in this way can be used foradjusting and monitoring the nozzle position of the cutting nozzle 6through which the laser beam 5 passes, specifically by means ofreference to the nozzle center P1. The nozzle center P1 is determinedfrom the circular nozzle (internal) contour K3 captured via the imagecapturing unit 10, the nozzle center P1 being determined as the centerof the circle of the latter. In this way, a distance A3 between thenozzle center P1 and the tool center point P2 can be determined. Inparticular, in the event of a deviation between nozzle center P1 andtool center point P2 being detected, action can be taken in the lasercutting process with the aid of the control apparatus 19 in order tocorrect the position of the laser beam 5 relative to the nozzle 6, sothat the tool center point P2 coincides with the nozzle center P1.

During the detection of the internal contour K3 of the nozzle 6, inparticular by using the geometric shape of the cutting nozzle,information can also be obtained about mechanical damage to the latter,specifically by deviations from the (typically circular) referencecontour of the nozzle 6 being detected. The damage can be produced,amongst other things, by a collision of the nozzle 6 with the workpiece2, by support webs (not shown) or other interfering contours or by ahigh level of laser beam eccentricity (in relation to the nozzle centerP1), in which the laser beam 5 touches the nozzle inner edge 6 a and asa result melts the latter locally. As a result of such damage, which cangenerally not be detected by the machine operator, the cutting gasdynamics and thus the cut quality can be changed detrimentally.

By using the illustrations of FIG. 2 and FIG. 1, the determination ofthe cutting front angle during a laser cutting process will be explainedbelow. In order to determine the cutting front angle, first a distanceA4 between the cutting front upper edge K2.1 and the cutting front loweredge K2.2 is determined which, as described above, are detected asmaterial boundaries with the aid of the evaluation apparatus 18. Thecutting front angle α shown in FIG. 1 is given by the trigonometricfunction α=arctan(A4/d) from the distance A4 between cutting front upperedge K2.1 and lower edge K2.2, measured along the gap center 25, andfrom the thickness d of the workpiece 2.

With the aid of the device 1 from FIG. 1, it is also possible todetermine a distance A5 between the cutting gas nozzle 6 and theworkpiece 2, more precisely the workpiece upper side 2 a. For thispurpose, the lens 12 installed upstream of the image capturing unit 10is displaced along the optical axis 11 with the aid of the positioningsystem 14, so that a distance between an image plane 10 a of the imagecapturing apparatus 10 for capturing the image 20 and the lens 12changes. At a first distance b1 between image plane 10 a and lens 12,the workpiece 2 and the upper side 2 a lies within the range of thedepth of focus of the image capturing apparatus 10, so that at least onematerial boundary of the workpiece 2, e.g. the cut edges K1.1, K1.2, canbe detected. At a second distance b2, the nozzle 6 lies in the range ofthe depth of focus of the image capturing apparatus 10, so that theevaluation apparatus 18 detects an internal contour K3 of the nozzle 6as material boundary. By using the difference b1−b2 between the twodistances b1, b2 at which the nozzle internal contour K3 and the cutedges K1.1, K1.2 are detected by the evaluation apparatus 18, thedistance A5 between the nozzle 6 and the workpiece 2 can be calculated.

FIG. 3 shows cut end detection when reaching or traveling over a metalsheet edge K4 at the outer edge of the workpiece 2. The position of thedetected metal sheet edge K4 in this case is related to the tool centerpoint P2, in order to determine a distance (not shown in FIG. 3) betweenthe tool center point P2 and the metal sheet edge K4. By means of theknowledge of this distance, the control device 19 can switch off thelaser beam 5 as soon as the latter reaches the metal sheet edge K4, sothat damage to the separated workpiece 2 when it falls down by a laserbeam 5 that is ignited for too long is avoided. Depending on theapplication, such a switch-off can also be performed before the metalsheet edge is reached by the tool center point P2, specifically as soonas the distance to the metal sheet edge K4 is sufficiently small inorder still to be able to implement complete separation of the workpiece2 despite switching off.

In a way analogous to FIG. 3, FIG. 4 shows cut end detection whenreaching an already cut contour, i.e. an already existing kerf 27. Thelaser beam 5 can be switched off in a way analogous to reaching ortraveling over the metal sheet edge K4 in FIG. 3, whose role in theimage 20 shown in FIG. 4 is performed by the first cut edge K5.1pointing in the direction of the cutting process.

Alternatively, via the detection of the two already present cut edgesK5.1, K5.2 of the further cut gap 27, the cut gap width A1 and, fromthis in turn, the end point P3 of the cut gap 24 running into the kerf27 can be defined. This end point P3 is typically arranged centrally,that is to say at the same distance (0.5 A1) from the opposite cut edgesK5.1, K5.2. Depending on the application, this end point P3 can,however, also be displaced in the direction of the cut edge K5.1 reachedfirst by the cut, for example when the incoming kerf 24 is larger thanthe kerf 27 running transversely thereto and already present. In theopposite case, i.e. when the incoming kerf 24 is smaller than the kerf27 running transversely thereto, a reverse displacement of the end pointP3 can likewise be beneficial, as long as the laser beam 5 is at leastswitched off in such a timely manner that damage to the rear cut edge5.2 of the further kerf 27 is avoided.

In general, by means of the detection of the relative position of analready generated kerf 27 in relation to a tool center point (TCP) P2found in the process, a path deviation can also be detected and, if atolerance range is exceeded, can be corrected with the aid of thecontrol device 19.

The case illustrated in FIG. 5 of the detection of the cut start iscarried out in a way comparable to the detection of the cut enddescribed in FIG. 3 and FIG. 4. As opposed to the cut end detection,however, the laser beam 5 is not switched off but switched on orconnected as a function of the (positional) relationship of tool centerpoint P2 to kerf 27 running transversely (or else at another angle). Thedetection of a cut start on a metal sheet edge K4 is carried out in away analogous to the procedure described in conjunction with FIG. 3. Cutstart detection is particularly suitable when resuming a cutting processfollowing a failed cut or else when resuming a cutting process after arelative movement between machining unit 3 and workpiece 2, for examplenecessitated by clock cycles during which the laser cutting process isinterrupted as needed by the process and it is necessary to start/resumeat exactly the same point again at a subsequent time.

FIG. 6 shows the image 20 of a region 21 of the workpiece 2 to bemonitored during a piercing operation, in which a circular hole 28 isintroduced into the workpiece 2. The two main influencing factors on thepiercing process, namely the chemical material composition and thesurface finish of the workpiece 2, can vary from manufacturer tomanufacturer and from batch to batch. During the piercing operation, inparticular in structural steel, e.g. with material thicknesses startingat 15 mm, problems may occur during the piercing operation on account ofthese differences in the material properties. The piercing operation inthick structural steel is disrupted in such a way that the laser beamdoes not drill a narrow hole but that, on account of the overheating andthe exothermic iron-oxygen reaction that subsequently proceeds, aconical crater is formed, the contour of which is shown in FIG. 6. Here,the evaluation apparatus 18 can detect the internal contour K6.1 of thehole 28 and the outer crater edge K6.2, so that looming craterproduction can be detected and the control apparatus 19 can initiatesuitable countermeasures, e.g. a cooling pause. The countermeasures canbe initiated, for example, when a limiting value for the distance A6between the inner boundary K6.1 and outer boundary K6.2 (crater edge) ofthe pierced hole 28 is exceeded. It is possible to conclude that thereis crater formation when the outer contour K6.2 of the pierced hole 28becomes so large that said contour disappears from the region 21captured by the camera 10 through the nozzle 6.

FIGS. 7A-C each show an image 20 of the process autoluminescence in theNIR/IR range of a region of interaction 31 in a melt cutting process,which image has been captured with the camera 10 by using a filter 13which was transparent only to process radiation 8 in the (near) infraredrange; the contours shown represent the boundaries between regions ofdifferent intensity of the process radiation 8 and the contours of theworkpiece 2 cannot be seen. The process radiation 8 registered by theimage 20 is autoluminescence of the laser cutting process, whichtypically comprises (at least to some extent) the melt bath. The image20 of the process autoluminescence cannot be equated directly with atemperature distribution, since the measured intensity I (cf. FIG. 1)depends on the temperature T substantially in accordance with thefollowing formula: I=ε*T⁴, where ε denotes the emissivity (between 0 and1). Since the emissivity ε in the present case can be close to zero, itis difficult to derive information about the temperature from theintensity distribution. Nevertheless, for the purpose of simplification,the measured intensity distribution will occasionally also be designateda thermal image below.

By using the (thermal) images 20, the formation of burrs or theirabsence during laser cutting, and thus the cut quality, can bedetermined. Here, FIG. 7A shows the image 20 of the region ofinteraction 31 in which a quality cut is being produced (with virtuallysmooth cut edges). The region of interaction 31 exhibits a singlecentral tail or a single luminous track 29 along the feed directionV_(sheet). In addition, when observing the wake of the region ofinteraction 31 over a relatively long time period (several seconds),sporadic flickering additionally occurs. The shape of the region ofinteraction 31 shown in FIG. 7A and the sporadic flickering (i.e. therepeating increase and decrease in the brightness) can be traced back tohomogenous melt expulsion which, in a quality cut, oscillates rearwardand forward in the feed direction. If no flickering can be detected,this is an indication of the formation of burrs during the cuttingprocess (and specifically of the presence of whisker burr).

FIG. 7B shows an image 20 of the region of interaction 31 in thepresence of burr formation, specifically during the formation of what isknown as crumb burr, in which, in the present example, two brightluminescent stripes 30 a, 30 b directed rearward from the cutting front23 can be seen at the two (not shown) cut edges, as well as a furtherluminescent stripe 30 c which runs in the middle between the two outerluminescent stripes 30 a, 30 b. The luminescent stripes 30 a-c here arecomparatively long, which points to the occurrence of an azimuthal meltflow with production of crumb burr far behind in the wake.

The image 20 of the region of interaction 31 shown in FIG. 7C likewisepoints to the formation of burr, specifically what is known as whiskerburr. In this case, no tail or luminescent stripe can be seen, since thecomplete melt volume feeds the whisker burr. In addition, no flyingsparks occur directly underneath the cutting gas nozzle 6, so that inthis case no flickering occurs; instead, the laser cutting processproceeds without noticeable fluctuations in the detected intensity ofthe process radiation 8.

FIG. 8 shows a thermal image 20 or an image of the processautoluminescence in the NIR/IR range, such as occurs in a structuralsteel flame cutting process (using oxygen as cutting gas). In such aprocess, the upper parts of the cut edges have periodically repeatinggrooves, which can be seen in the thermal image 20 as striations 33. Inthe region of the cutting front 23, a local minimum 32 with a radiationintensity that is reduced as compared with the intensity in thesurroundings occurs in the image 20 of the region of interaction 31 whena good cut is present, i.e. without burr formation. The size of the areaF1 of the intensity minimum 32 (radiation sink) can be monitored and,for the case in which the latter decreases too sharply, burr formationcan be counteracted by the process parameters being changed suitably.

By using the thermal image from FIG. 8, imminent material burn-up canalso be detected. Here, use can be made of the fact that the frequency fof the striations 33 in the thermal image 20 of the region ofinteraction 31 decreases in the region of that cut edge at whichmaterial burn-up is imminent, so that suitable countermeasures can betaken in order to suppress the occurrence of the material burn-up. Thestriations 33 or a decrease in the frequency f of the striations canalternatively also be detected in the visible range.

In addition, material burn-up that has already arisen and/or is justimminent can be detected by using a rise in the brightness of theoverall intensity of the thermal image 20, since the luminescent areaobserved through the nozzle 6 increases in the event of materialburn-up. Additionally or alternatively, material burn-up can also bedetected by an increased fluctuation in the overall brightness value ascompared with a conventional cutting process.

The thermal images 20 can be compared with the material boundaries(contours of the workpiece 2) detected (at wavelengths in the visiblerange), in order to improve the determination of values characteristicof the laser cutting process. Here, in particular in the case ofworkpieces made of stainless steel, a failed cut can be detected whenthe width of the luminous area registered in the thermal image, whichcorresponds substantially to the width of the cutting front, is greaterthan the width A2 of the cut gap at right angles to the feed directionV_(sheet) (cf. FIG. 2).

Both the capturing of the material boundaries and the capturing of thethermal image of the region of interaction in the apparatus shown inFIG. 1 are carried out with the aid of a single camera as imagecapturing apparatus. For this purpose, the wavelength filter 13 issuitably tuned or suitably moved into the beam path of the process light8 and out again. For the purpose of parallel detection of materialboundaries and thermal image, the image capturing apparatus 10 can alsohave further cameras and/or detectors.

A number of embodiments have been described. Nevertheless, it will beunderstood that various modifications may be made without departing fromthe spirit and scope of the invention. Accordingly, other embodimentsare within the scope of the following claims.

What is claimed is:
 1. A method for monitoring a laser cutting processon a workpiece, the method comprising: capturing, by an image capturingapparatus, an image of a region of the workpiece to be monitored,detecting, by an evaluation apparatus, material boundaries of theworkpiece using the captured image, wherein the detected materialboundaries include a cutting front upper edge of a workpiece surfacefacing an incident laser beam and a cutting front lower edge of aworkpiece surface facing away from the incident laser beam; anddetermining, by the evaluation apparatus, a cutting front angle of thelaser cutting process based on a geometric relationship between at leasttwo of the detected material boundaries, the at least two of thedetected material boundaries including the cutting front upper edge andthe cutting front lower edge.
 2. The method of claim 1, whereindetermining the cutting front angle comprises determining a distance Aas the geometric relationship between the cutting front upper edge andthe cutting front lower edge.
 3. The method of claim 1, furthercomprising: determining, by the evaluation apparatus, the cutting frontangle based on the geometric relationship between the cutting frontupper edge and the cutting front lower edge taking into account athickness of the workpiece.
 4. The method of claim 3, wherein thecutting front angle α is determined byα=arctan(A/d), wherein A represents a distance between the cutting frontupper edge and the cutting front lower edge and d represents thethickness of the workpiece.
 5. The method of claim 1, furthercomprising: adjusting, by a control apparatus, a cutting speed of thelaser cutting process as a function of the cutting front angle.